45 research outputs found
Initial conditions for cosmological perturbations in scalar-tensor dark-energy models
We discuss the evolution and imprints of isocurvature initial conditions for the scalar field in scalar tensor extensions of Einstein gravity. We consider the simplest models of scalar tensor theories, as Induced gravity (IG, which can be recasted in form of Jordan-Brans-Dicke theory by a redefinition of the scalar field) or non-minimally coupled (NMC) scalar fields in which the acceleration of the Universe is connected to a variation of the effective Planck mass. After introducing the fundamental ideas of cosmological perturbation theory and scalar tensor theories of gravity, we give the evolution equations for matter, metric and scalar field fluctuations in synchronous gauge. We use this set of equations for both the IG and the NMC models to find a new isocurvature solution in which the scalar field fluctuations compensate for the relativistic components respectively. We also show how we can generalize the well known isocurvature modes in Einstein GR to these models. We show the different evolution of cosmological fluctuations for these isocurvature initial conditions compared to the standard adiabatic one. After that, we compute the CMB angular power spectrum for these solutions in the IG model, with the help of a modified Einstein-Boltzmann CLASS code. In particular the CMB power spectrum is computed separately for adiabatic and isocurvature initial conditions, i.e. for totally uncorrelated modes, and with arbitrary correlations leading to an interesting explanation of the lack of power in the low multipoles region of the CMB temperature power spectrum. Finally we show how a simple model of double inflation in IG could explain the generation of the new isocurvature mode
Questions on calculation of primordial power spectrum with large spikes: the resonance model case
Inflationary models predicting a scale-dependent large amplification of the
density perturbations have recently attracted a lot of attention because the
amplified perturbations can seed a sizable amount of primordial black holes
(PBHs) and stochastic background of gravitational waves (GWs). While the power
spectra in these models are computed based on the linear equation of motion, it
is not obvious whether loop corrections are negligible when such a large
amplification occurs during inflation. In this paper, as a first step to
discuss the loop corrections in such models, we use the in-in formalism and
calculate the one-loop scalar power spectrum numerically and analytically in an
illustrative model where the density perturbations are resonantly amplified due
to oscillatory features in the inflaton potential. Our calculation is
technically new in that the amplified perturbations are numerically taken into
account in the in-in formalism for the first time. In arriving at our
analytical estimates, we highlight the role that the Wronskian condition of
perturbations, automatically satisfied in our model, plays in obtaining the
correct estimates. In addition, the analytical estimates show that the
contribution originating from the quantum nature of the perturbations in the
loop can be dominant. We also discuss the necessary conditions for subdominant
loop corrections in this model. We find that, for the typical parameter space
leading to the amplification of the power spectrum required
for a sufficient PBH production, the one-loop power spectrum dominates over the
tree-level one, indicating the breakdown of the perturbation theory.Comment: 42 pages, 15 figure
Scalar-tensor theories in light of cosmological tensions
In this PhD thesis, I study cosmologies within the simplest scalar-tensor theories of gravity consisting in a scalar field non-minimally coupled to the Ricci scalar through a function that induces a time-variation in the Newton constant and a potential . I explore the new physics in these cosmologies and use publicly available data to constrain them.
Depending on the functional form of the non-minimal coupling and the potential, the cosmological dynamics changes significantly. For some of the models, the specific dynamics helps recover the consistency with very stringent tests of General Relativity from Solar System and laboratory experiments without the need of any screening mechanisms.
When compared to publicly available data, all these models feature a value of the Hubble constant larger than the standard CDM cosmology. This makes scalar-tensor theories one of the most interesting candidates to solve the tension which is becoming one of the most pressing questions in the post-Planck cosmology.
In order to better characterize the phenomenology of scalar-tensor theories, I also investigate their degeneracy with parameters describing the physics of neutrinos. I show that bounds on the effective number of active neutrinos and their masses are slightly relaxed in this context, although they are only a very weakly degenerate with the modification to gravity studied in this thesis and the full inclusion of CMB and
LSS data used here.
Finally, I address the issue of
initial conditions within these theories and present a new regular isocurvature mode connected with the variation of the Newton constant which is absent in Einstein gravity. Although the observational imprints are different, the allowed fraction of this mode, relative to the adiabatic one, is constrained by Cosmic Microwave Background data at a similar level to other known isocurvature modes
Uncovering the History of Cosmic Inflation from Anomalies in Cosmic Microwave Background Spectra
We propose an inflationary primordial feature model that can explain both the
large and small-scale anomalies in the currently measured cosmic microwave
background (CMB) anisotropy spectra, revealing a clip of adventurous history of
the Universe during its primordial epoch. Although the model is currently
statistically indistinguishable from the Standard Model, we show that future
observations such as the Simons Observatory and LiteBIRD will complement each
other in distinguishing the model differences due to their accurate E-mode
polarization measurements, and the PICO mission, if funded, can put stringent
constraints on all characteristic properties. The model predicts a signal of
classical primordial standard clock, which can also be used to distinguish the
inflation and alternative scenarios in a model-independent fashion.Comment: 8 pages, 4 figure
Hybrid -attractors, primordial black holes and gravitational wave backgrounds
We investigate the two-stage inflation regime in the theory of hybrid
cosmological -attractors. The spectrum of inflationary perturbations is
compatible with the latest Planck/BICEP/Keck results, thanks to the attractor
properties of the model. However, at smaller scales, it may have a very high
peak of controllable width and position, leading to a copious production of
primordial black holes (PBH) and generation of a stochastic background of
gravitational waves (SGWB).Comment: 39 pages, 12 figure
Early modified gravity in light of the tension and LSS data
We present a model of Early Modified Gravity (EMG) consisting in a scalar
field with a non-minimal coupling to the Ricci curvature of the type
plus a cosmological constant and a small effective
mass and demonstrate its ability to alleviate the tension while providing
a good fit to Cosmic Microwave Background (CMB) anisotropies and Baryon
Acoustic Oscillations (BAO) data. In this model the scalar field, frozen deep
in the radiation era, grows around the redshift of matter-radiation equality
because of the coupling to non-relativistic matter. The small effective mass,
which we consider here as induced by a quartic potential, then damps the scalar
field into coherent oscillations around its minimum at , leading to a
weaker gravitational strength at early times and naturally recovering the
consistency with laboratory and Solar System tests of gravity. We analyze the
capability of EMG with positive to fit current cosmological observations
and compare our results to the case without an effective mass and to the
popular early dark energy models with . We show that EMG with a quartic
coupling of the order of can
substantially alleviate the tension also when the full shape of the
matter power spectrum is included in the fit in addition to CMB and Supernovae
(SN) data.Comment: 32 pages, 11 figures; v2, 26 pages, 11 figures, minor changes,
accepted for publication in PR
Tracking the origin of black holes with the stochastic gravitational wave background popcorn signal
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record Monthly Notices of the Royal Astronomical Society 519.4 (2023): 6008-6019
is available online at: https://academic.oup.com/mnras/article-abstract/519/4/6008/6985653Unresolved sources of gravitational waves (GWs) produced by the merger of a binary of black holes at cosmological distances combine into a stochastic background. Such a background is in the continuous or popcorn regime, depending on whether the GW rate is high enough so that two or more events overlap in the same frequency band. These two regimes respectively correspond to large and small values of the so-called duty cycle. We study the detection regime of the background in models of primordial black holes (PBHs) and compare it to the one produced by black holes of stellar origin. Focusing on ground-based detectors, we show that the duty cycle of the PBH-origin background is larger than that of astrophysical black holes because of differences in their mass function and the merger rate. Our study opens up the possibility to learn about the primordial or astrophysical nature of black hole populations by examining the statistical properties of the stochastic backgroundThis work is supported by the Spanish Research Projects PGC2018-094773-B-C32 (MINECO) and PID2021-123012NB-C43 (MICINN-FEDER) and the Centro de Excelencia Severo Ochoa Program CEX2020-001007-S. MB and SK are supported by the Spanish Atracción de Talento contract no. 2019-T1/TIC-13177 granted by Comunidad de Madrid, the I+D grant PID2020-118159GA-C42 of the Spanish Ministry of Science and Innovation and the i-LINK 2021 grant LINKA20416 of CSIC. SK is partially supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant no. 20H01899 and 20H0585